Fractures of the radial head constitute one of the two most common fractures of the elbow, and treatment of radial head fractures has been a controversial subject. Although excision of the head of the radius can sometimes give good long-term results, such may often cause persistent elbow or wrist pain associated with cubitus valgus or with proximal radial migration. As a result, prosthetic replacements have been frequently recommended to obviate these problems and to improve stability of the elbow joint.
Radial heads have sometimes been replaced with flexible silicone rubber prostheses, particularly in cases of severe rheumatoid arthritis and degenerative arthritis, and such replacements have been suggested for comminuted fractures of the radial head. However, because a silicone rubber radial head is quite flexible, compared to bone, the ability of such a flexible implant to transfer dynamic forces across the radial-capitellar joint without excessive deformation is questioned. Clinical and biomechanical studies have now indicated that replacement of a radial head with a prosthesis made of silicone rubber, which is much less stiff than bone and deforms under load, does not reestablish the true mechanical function of the natural radial head because such a silicone rubber prosthesis may be unable to transmit physiological forces from the proximal radius to the capitellum; this indicates that a less flexible radial head prosthesis should result in more normal physiologic stress transfers and provide improved clinical results. Moreover, fractures of silicone rubber devices have also been reported.
Both metal and acrylic radial head replacements have now been clinically implanted. Although there is only limited experience with prostheses made of metal, the elastic moduli of certain metals, such as titanium, can be as much as 10 times greater than the elastic modulus of bone, and this very substantial difference often results in a biomechanical incompatibility between the bone of the capitulum and the radial head replacement prosthesis. Moreover, the difference has also been such as to cause undesirable changes in cartilage in situations where there is cartilage at the wear surface.
Shown in U.S. Pat. No. 5,782,923 is one endoprosthesis for replacing the entire elbow joint wherein a radius component is mounted in the bone by a shaft portion and has a spherical head that is received in a recess of a member that slides in a socket formed in the undersurface of a flange portion of the ulnar component. U.S. Pat. No. 5,030,237 shows an elbow prosthesis which includes a humeral insert made of a metal alloy, such as Vitallium, and an ulna prosthesis, wherein the end of the radius may be optionally replaced.
Although the results of certain metal and polymeric implants have been promising in some aspects, none of these replacements have been totally satisfactory. Accordingly, the search has continued for more satisfactory radial head replacements that better mimic the properties of the natural radius head.